Delay stage-interweaved analog DLL/PLL

ABSTRACT

A methodology is disclosed that enables the delay stages of an analog delay locked loop (DLL) or phase locked loop (PLL) to be programmed according to the operating condition, which may depend on the frequency of the input reference clock. The resulting optimized delay stages allow for a broad frequency range of operation, fast locking time over a wide range of input clock frequencies, and a lower current consumption at high clock frequencies. Better performance is achieved by allowing the number of analog delay stages active during a given operation to be flexibly set. The deactivation or turning off of unused delay stages conserves power at higher frequencies. The high frequency range of operation is increased by using a flexible number of delay stages for various input clock frequencies. Because of the rules governing abstracts, this abstract should not be used to construe the claims.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/893,804entitled Delay Stage-Interweaved Analog DLL/PLL filed Jul. 19, 2004 nowU.S. Pat. No. 7,149,145.

BACKGROUND

1. Field of the Disclosure

The present disclosure generally relates to memory systems and, moreparticularly, to an analog delay locked loop (DLL) or phase locked loop(PLL) with delay stage interweaving.

2. Brief Description of Related Art

Most digital logic implemented on integrated circuits is clockedsynchronous sequential logic. In electronic devices such as synchronousdynamic random access memory circuits (SDRAMs), microprocessors, digitalsignal processors, etc., the processing, storage, and retrieval ofinformation is coordinated or synchronized with a clock signal. Thespeed and stability of the clock signal determines to a large extent thedata rate at which a circuit can function. Many high speed integratedcircuit devices, such as SDRAMs, microprocessors, etc., rely upon clocksignals to control the flow of commands, data, addresses, etc., into,through and out of the devices.

In SDRAMs or other semiconductor memory devices, it is desirable to havethe data output from the memory synchronized with the system clock thatalso serves a microprocessor. Delay-locked loops (DLLs) are synchronouscircuits used in SDRAMs to synchronize an external clock (e.g., thesystem clock serving the microprocessor) and an internal clock (e.g.,the clock used internally within the SDRAM to perform data read/writeoperations on various memory cells) with each other. Typically, a DLL isa feedback circuit that operates to feed back a phase difference-relatedsignal to control a delay line, until the timing of one clock signal(e.g., the system clock) is advanced or delayed until its rising edge iscoincident or has a fixed time delay relationship (or “locked”) with therising edge of a second clock signal (e.g., the memory's internalclock).

FIG. 1 is a simplified block diagram showing a memory chip or memorydevice 12. The memory chip 12 may be part of a DIMM (dual in-line memorymodule) or a PCB (printed circuit board) containing many such memorychips (not shown in FIG. 1). The memory chip 12 may include a pluralityof pins 14 located outside of chip 12 for electrically connecting thechip 12 to other system devices. Some of those pins 14 may constitutememory address pins or address bus 17, data pins or data bus 18, andcontrol pins or control bus 19. It is evident that each of the referencenumerals 17-19 designates more than one pin in the corresponding bus.Further, it is understood that the schematic in FIG. 1 is forillustration only. That is, the pin arrangement or configuration in atypical memory chip may not be in the form shown in FIG. 1.

A processor or memory controller (not shown) may communicate with thechip 12 and perform memory read/write operations. The processor and thememory chip 12 may communicate using address signals on the addresslines or address bus 17, data signals on the data lines or data bus 18,and control signals (e.g., a row address strobe (RAS) signal, a columnaddress strobe (CAS) signal, etc. (not shown)) on the control lines orcontrol bus 19. The “width” (i.e., number of pins) of address, data andcontrol buses may differ from one memory configuration to another.

Those of ordinary skill in the art will readily recognize that memorychip 12 of FIG. 1 is simplified to illustrate one embodiment of a memorychip and is not intended to be a detailed illustration of all of thefeatures of a typical memory chip. Numerous peripheral devices orcircuits may be typically provided along with the memory chip 12 forwriting data to and reading data from the memory cells 20. However,these peripheral devices or circuits are not shown in FIG. 1 for thesake of clarity.

The memory chip 12 may include a plurality of memory cells 20 generallyarranged in rows and columns to store data in rows and columns. Eachmemory cell 20 may store a bit of data. A row decode circuit 22 and acolumn decode circuit 24 may select the rows and columns in the memorycells 20 in response to decoding an address, provided on the address bus17. Data to/from the memory cells 20 is then transferred over the databus 18 via sense amplifiers and a data output path (not shown). A memorycontroller (not shown) may provide relevant control signals (not shown)on the control bus 19 to control data communication to and from thememory chip 12 via an I/O (input/output) unit 26. The I/O unit 26 mayinclude a number of data output buffers (not shown) to receive the databits from the memory cells 20 and provide those data bits or datasignals to the corresponding data lines in the data bus 18. The I/O unit26 may further include a clock synchronization unit or delay locked loop(DLL) 28 to synchronize the external system clock (e.g., the clock usedby the memory controller (not shown) to clock address, data and controlsignals between the memory chip 12 and the controller) with the internalclock used by the memory 12 to perform data write/read operations on thememory cells 20. In the embodiment of FIG. 1, the DLL 28 is an analogDLL, which is described in more detail below with reference to FIG. 2.

The memory controller (not shown) may determine the modes of operationof memory chip 12. Some examples of the input signals or control signals(not shown in FIG. 1) on the control bus 19 include an External Clocksignal, a Chip Select signal, a Row Access Strobe signal, a ColumnAccess Strobe signal, a Write Enable signal, etc. The memory chip 12communicates to other devices connected thereto via the pins 14 on thechip 12. These pins, as mentioned before, may be connected toappropriate address, data and control lines to carry out data transfer(i.e., data transmission and reception) operations.

FIG. 2 depicts a simplified block diagram of the analog delay-lockedloop (DLL) 28 shown in FIG. 1. The analog DLL 28 is a 4-phase DLL,generating the Ph0, Ph90, Ph180, and Ph270 signals at its output 33. Onthe other hand, a 2-phase analog DLL would generate, for example, a Ph0and a Ph180 signals only. The DLL 28 receives a reference clock (ClkREF)46 as an input and generates a set of output clock signals (Ph0, Ph90,Ph180, Ph270, Ph360) at the output 33 of a voltage controlled delay line(VCDL) 32. The Ph0 and Ph360 signals are, in turn, fed back into a phasedetector 30 whose operation is discussed below. In the discussionherein, the notation “Ph0” is used to refer to a clock signal that issubstantially in phase with the reference clock 46, whereas the “Ph360”signal is substantially 360° out of phase with ClkREF 46. Similarly, thePh90 clock signal is substantially 90° out of phase with ClkREF 46, thePh180 clock is substantially 180° out of phase with ClkREF 46, and Ph270clock is substantially 270° out of phase with the reference clock 46. Itis noted that the reference clock 46 is interchangeably referred toherein as “ClkREF”, “ClkREF signal”, “Ref clock signal”, “Ref clock” or“system clock”; whereas each of the various output clocks (Ph0, Ph90,Ph180, etc.) is individually referred to herein as a “phase signal” andcollectively as “phase signals.” The reference clock 46 is typically theexternal system clock serving the microprocessor (or memory controller)(both not shown) or a delayed/buffered version of the external systemclock.

One or more of the output phase signals Ph0, Ph90, etc., or signalsderived from them, may be used as “internal clock(s)” by the SDRAM 12 toperform data read/write operations on memory cells 20 and to transferthe data out of the SDRAM to the data requesting device (e.g., amicroprocessor (not shown)). As can be seen from FIG. 2, the phasesignals are generated using delay lines (not shown) in the VCDL 32,which introduces a specific delay into the input Ref clock 46 to obtainthe “lock” condition—i.e., to obtain specific output clocks or phasesignals (Ph0, Ph90, etc.) having a predetermined phase relationship withthe input reference clock 46. The phase detector (PD) 30 compares therelative timing of the Ph0 and Ph360 phase signals (both of which relateto the reference clock 46 in a determined manner) to generate one of apair of direction signals—the UP signal 34 or the DN (down) signal35—depending on the phase difference between the Ph0 and Ph360 signals.The direction signal outputs are fed to a charge pump 36, whichgenerates a control voltage signal Vctrl 38 whose value at a giveninstant in time depends on the inputs received from the phase detector30. Thus, the voltage level of the control voltage Vctrl 38 isrepresentative of the phase difference between the Ph0 and Ph360 phasesignals and, hence, between the ClkREF signal 46 and its 360° delayedversion. The control voltage signal 38 is fed to a bias generator 40,which generates a pair of bias voltage outputs or bias signals—a PMOS(p-channel metal oxide semiconductor) bias voltage VBP 42 and an NMOS(n-channel MOS) bias voltage VBN 43—based on the voltage level of theinput Vctrl signal 38. For example, in one embodiment, the PMOS biasvoltage VBP may be substantially equal to or may vary directly withVctrl, the control input to the bias generator. In that case, when thevalue of Vctrl goes high, the value of VBP goes high whereas the valueof VBN goes low. And, when the value of Vctrl goes low, the value of VBPalso goes low proportionately whereas the value of VBN goes high.

The bias voltages are applied to the VCDL unit 32 to control the delayimparted therein to the reference clock 46 input thereto. In oneembodiment, when VBP goes high and VBN goes low, the delay imparted byVCDL 32 increases; whereas, when VBP goes low and VBN goes high, thedelay decreases. Although a single output line 33 is illustrated in FIG.2, the VCDL 32 may have separate output lines (not shown) to output eachof the phase signals Ph0, Ph90, etc., individually. It is noted herethat additional constructional details or circuit details (of individualcircuit units, e.g., the charge pump 36 or the bias generator 40) forthe analog DLL 28 in FIG. 2 is not provided herein for the sake ofbrevity and also because such details are known to one skilled in theart.

As noted before, the analog DLL 28 in FIG. 2 is a 4-phase DLL, which maybe employed when the external or system clock 46 has a frequency (e.g.,800 MHz) that substantially differs from the frequency (e.g., 400 MHz)of the memory's internal clock (not shown). On the other hand, if theinternal and external frequencies are almost equal (e.g., both equal to800 MHz), then a 2-phase DLL (generating only Ph0 and Ph180 outputs) maysuffice as is known in the art.

It is observed that the frequency range of operation of the DLL 28(i.e., the available range of delay) is dependent on the range of Vctrl38, the gain of various circuit elements in the DLL 28, the number ofvarious VCDL stages constituting the VCDL unit 32, and the PVT (process,voltage, temperature) variations during circuit fabrication and at runtime. Generally, if too many VCDL stages (not shown) are used for lowfrequency operation, the analog DLL 28 requires a lot of current at highfrequency because some of those delay stages in VCDL 32 may beunnecessarily kept turned ON during high frequency operation. At highreference clock frequencies, it may not be preferable to increase thenumber of VCDL stages because that may also increase the correspondingoverall delay. However, on the other hand, if less than optimum numberof VCDL stages are employed, the VCDL may not properly function at lowinput clock frequencies. For example, if delay at each VCDL stage (notshown) is in the range of 300-700 ps (picoseconds) (a range of delay isavailable because of the voltage-controlled nature of the VCDLoperation), then four (4) VCDL stages may be needed to obtain a delayrange of 1.2 ns-2.8 ns for a low frequency operation. However, with thesame number (4) of VCDL stages, it may not be possible to obtain a delayrange of 1 ns-4 ns which may be needed to accommodate a higher referenceclock frequency (e.g., a frequency having a clock period t_(CK)=1 ns).On the other hand, if the number of VCDL stages are reduced to three (3)to obtain the delay range of 0.9 ns-2.1 ns (so as to accommodate theminimum clock period t_(CK) of ins), then the reduced number of delaystages would fail to accommodate lower clock frequencies having periodsin the range of 2.1 ns-2.8 ns.

It is noted here that the discussion presented hereinabove equallyapplies to an analog phase-locked loop (PLL) that may be used in placeof the analog DLL 28 in the memory chip 12 as is known in the art. ThePLL implementation may include a VCO (Voltage Controlled Oscillator)instead of the VCDL 32 for the DLL version. However, the VCDL and VCOmay be generally considered as voltage-controlled frequency monitoringunits. Because of substantial similarity in the construction andoperation of an analog PLL and an analog DLL, only the DLLimplementation is discussed herein. However, it is evident that theentire DLL-related discussion presented herein equally applies to aPLL-based embodiment, of course with suitable PLL-specific modificationsas may be apparent to one skilled in the art.

SUMMARY

The inventors have recognized that the frequency range of operation,locking time, and current consumption (e.g., at higher frequencies) of aprior art analog delay locked loop may be negatively affected because ofthe usage of all delay line stages for all frequencies of operation.Thus, the number of delay stages active in the delay line is fixedregardless of the frequency of the reference clock signal. Thisarrangement not only consumes extra current (and, hence, power) athigher frequencies of operation, but also results in inefficient andinflexible usage of the delay line. The usage of the entire delay linefor each frequency delay operation results in slower locking time and anarrow frequency range of operation. Therefore, the present disclosurecontemplates an analog delay locked loop (or phase locked loop) whereindelay stages in a voltage-controlled delay line (or voltage-controlledoscillator) may be programmed according to the operating condition(e.g., the frequency of the input reference clock).

In one embodiment, the present disclosure contemplates a method ofoperating a synchronous circuit. The method comprises: applying areference clock as an input to a voltage controlled delay line (VCDL) inthe synchronous circuit, wherein the VCDL includes a plurality of delaystages to delay the reference clock input to the VCDL; obtaining aplurality of output signals at an output of the VCDL; and disabling oneor more of the plurality of delay stages using a first set of two ormore of the plurality of output signals.

In another embodiment, the present disclosure contemplates a method ofoperating a synchronous circuit, wherein the method comprises: obtaininga reference clock; generating a plurality of output signals, whereineach of the plurality of output signals has a respective phaserelationship with the reference clock; and using a first subset of theplurality of output signals to determine which one or more of aplurality of delay stages in the synchronous circuit are to be appliedto the reference clock.

In a further embodiment, the present disclosure contemplates anothermethod of operating a synchronous circuit. The method comprises:applying a reference clock as an input to a voltage controlled delayline (VCDL) in the synchronous circuit, wherein the VCDL includes aplurality of delay stages to delay the reference clock input thereto;generating a plurality of output signals at an output of the VCDL,wherein each of the plurality of output signals has a respective phaserelationship with the reference clock; and selectively maintaining oneor more of the plurality of delay stages activated based on a phaserelationship between the reference clock and each output signal in asubset of the plurality of output signals.

The present disclosure also contemplates a synchronous circuit and asystem including a memory chip containing the synchronous circuit toperform various method steps outlined above.

The programming of the delay stages according to the operating conditionmay result in optimized delay stages that allow for broad frequencyrange of operation, fast locking time over a wide range of input clockfrequencies, and a lower current consumption at high clock frequencies.Better performance may be achieved by allowing the number of analogdelay stages active during a given operation to be flexibly set. Thedeactivation or turning off of unused delay stages may conserve power athigher frequencies. The high frequency range of operation may beincreased because of the removal of the prior art restriction of using afixed number of delay stages for all input clock frequencies.

BRIEF DESCRIPTION OF THE DRAWING

For the present disclosure to be easily understood and readilypracticed, the present disclosure will now be described for purposes ofillustration and not limitation, in connection with the followingfigures, wherein:

FIG. 1 is a simplified block diagram showing a memory chip or memorydevice;

FIG. 2 depicts a simplified block diagram of the analog delay-lockedloop (DLL) shown in FIG. 1;

FIG. 3 illustrates an exemplary block diagram of an analog DLL accordingto one embodiment of the present disclosure;

FIG. 4 shows exemplary constructional details of the voltage-controlleddelay line (VCDL) used in the analog DLL illustrated in FIG. 3;

FIG. 5 depicts how respective phase signals are generated and input tothe phase detector in a 2-phase analog DLL according to one embodimentof the present disclosure;

FIG. 6 is an exemplary timing diagram illustrating a timing relationshipamong a set of low frequency output signals and the Vctrl signal in theanalog DLL of FIG. 3;

FIG. 7 is an exemplary timing diagram depicting a timing relationshipamong a set of high frequency output signals and the Vctrl signal in theanalog DLL of FIG. 3;

FIG. 8 depicts how respective phase signals are generated and input tothe phase detector in a 4-phase analog DLL according to one embodimentof the present disclosure;

FIG. 9 is an exemplary circuit layout illustrating how eight enablesignals may be generated using outputs from a frequency to delaydetector (FDD) to enable/disable each delay stage in a VCDL according toone embodiment having eight delay stages;

FIG. 10 shows an exemplary circuit layout of an FDD according to oneembodiment of the present disclosure;

FIG. 11 depicts an exemplary circuit layout according to one embodimentof present disclosure showing how a Ph180 phase signal may be generatedusing outputs of an FDD;

FIG. 12 shows a table listing a set of clock timing ratios andcorresponding values of FDD outputs according to one embodiment of thepresent disclosure; and

FIG. 13 is a block diagram depicting a system in which the analog DLL ofFIG. 3 may be used.

DETAILED DESCRIPTION

Reference will now be made in detail to certain embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying figures. It is to be understood that the figures anddescriptions of the present disclosure included herein illustrate anddescribe elements that are of particular relevance to the presentdisclosure, while eliminating, for the sake of clarity, other elementsfound in typical data storage or memory systems. It is noted at theoutset that the terms “connected”, “connecting,” “electricallyconnected,” etc., are used interchangeably herein to generally refer tothe condition of being electrically connected. It is further noted thatvarious block diagrams, circuit diagrams and timing waveforms shown anddiscussed herein employ logic circuits that implement positive logic,i.e., a high value on a signal is treated as a logic “1” whereas a lowvalue is treated as a logic “0.” However, any of the circuit discussedherein may be easily implemented in negative logic (i.e., a high valueon a signal is treated as a logic “0” whereas a low value is treated asa logic “1”).

As noted before, because of substantial similarities (in construction,operation, as well as application) between an analog DLL and an analogPLL (both of which contain voltage-controlled frequency monitoring unitsas noted hereinbefore), only the DLL implementation is discussed herein.However, it is evident that the entire DLL-related discussion presentedherein equally applies to a PLL-based embodiment, of course withsuitable PLL-specific modifications as may be apparent to one skilled inthe art.

FIG. 3 illustrates an exemplary block diagram of an analog DLL 48according to one embodiment of the present disclosure. The DLL 48 may beused in place of the prior art DLL 28 in the memory chip 12. It is notedhere that circuit elements that are functionally common between the DLL28 and the DLL 48 are indicated by the same reference numerals in FIGS.2 and 3. Thus, upon comparison of FIGS. 2 and 3, it is seen that the DLL48 contains a voltage-controlled delay line (VCDL) 50 that isfunctionally different from the VCDL unit 32 in FIG. 2. Although theVCDL unit 50 receives the same reference clock 46 and the same biassignal (which includes a pair of bias voltages VBP 42 and VBN 43) asinputs thereto as in the embodiment of FIG. 2, the generation and usageof output phase signals at the output 52 of VCDL unit 50 is different.The output phase signals Ph<0:n> contain many more outputs than the2-phase or 4-phase outputs discussed hereinbefore with reference to FIG.2. In the embodiment of FIG. 3, the output 52 of VCDL 50 contains n+1phase signals Ph<0:n>—each corresponding to a different one of the n+1delay stages (not shown in FIG. 3, but shown in FIG. 4) in the VCDL unit50. Thus, each delay stage in the VCDL unit 50 generates a correspondingoutput in the embodiment of FIG. 3, and one or more of such output phasesignals Ph<0:n> are used, as discussed later hereinbelow, to generatePh0 and Ph360 phase signals that are input to the phase detector 30 asalready discussed hereinbefore. It is noted here that although a singleoutput line 52 is illustrated in FIG. 3, the output of the VCDL 50 mayinclude n+1 output lines—each carrying a corresponding one of the n+1phase signals Ph<0:n>. The numeral “52” is used for the sake of clarityto collectively illustrate all such outputs of the VCDL unit 50.

It is observed here that although the functionality of various circuitelements common between the implementations of FIGS. 2 and 3 (e.g., thephase detector 30, the charge pump 36, etc.) may be similar, the circuitelement themselves may not be identically constructed. For example, thephase detector 30 in the embodiment of FIG. 3 may have constructiondifferent from or may be a modified version of the phase detector 30 inFIG. 2 to accommodate the design and signal considerations that arespecific to the DLL 48 embodiment of FIG. 3. For example, the biasgenerator 40 in FIG. 3 may include a bypass capacitor between VBN line43 and ground to provide additional stability at high frequencyoperations. The charge pump 36 in FIG. 3 may be a differential chargepump (to avoid charge sharing) and may also be modified from that inFIG. 2 to include a differential low pass filter for higher DC offsetcancellation. Additional modifications to these and other circuitelements in FIG. 3 may also be contemplated depending on the designconsiderations for the design of the analog DLL 48

FIG. 4 shows exemplary constructional details of the voltage-controlleddelay line (VCDL) 50 used in the analog DLL 48 illustrated in FIG. 3.The VCDL 50 in FIG. 3 receives the VBP 42, VBN 43, and ClkREF 46 signalsas inputs (similar to the VCDL 32 in FIG. 2), which are connected toeach of the delay stages 54-58 constituting the VCDL unit 50 as shown inFIG. 4. However, VCDL 50 selectively applies (as discussed laterhereinbelow) one or more of its constituent delay stages 54-58 to thereference clock 46 (based on corresponding Enable signals discussedlater hereinbelow with reference to FIG. 9) to generate appropriateoutput signals—i.e., one or more of the Ph<0:n> outputs are selectivelygenerated depending on the operating condition of VCDL 50. A delay stage54-58 is considered “applied” to the reference clock 46 (to impartcorresponding delay to the reference clock) when that delay stage isturned ON or electrically activated to perform its delay generationfunction. Thus, even though the reference clock line 46 is shownconnected to each delay stage 54-58 in FIG. 4, one or more of the delaystages 54-58 in FIG. 4 may remain inactive or turned OFF (and, hence,not “applied” to the ClkREF signal 46) depending on the operationcondition of VCDL 50 as discussed below.

The selective activation of delay stages in the VCDL 50 of FIGS. 3 and 4differs from the non-selective usage of all VCDL stages for allfrequencies of operation in the prior art VCDL unit 32 discussedhereinbefore with reference to FIG. 2. The controlled application (oractivation) in FIG. 4 of only those VCDL stages 54-58 that are neededfor a given reference clock frequency allows for a broader frequencylocking range, faster analog DLL locking time, and lower currentconsumption (and, hence, lower power consumption) at high frequencies.

It is seen from FIG. 4 that each output signal Ph<0>, Ph<1>, . . . ,Ph<n> has the same frequency but different phase from the inputreference clock 46. That is, each output signal is a correspondinglydelayed version of the reference clock 46, the delay being determined bythe position of the delay stage generating the respective output signal.Each delay stage provides one unit of delay (may be denoted as “t_(D)”),which is implied in FIG. 4 by the letters “VCDL1.” Although each delaystage may provide a range of delays (e.g., from 300 ps-700 ps), the unitdelay for a given clock reference frequency may have a single value inthat range of delays (the value being dependent on the frequency ofClkREF signal 46). In one embodiment, for example, the first delay stage54 may be configured not to provide the unit delay to the inputreference clock 46. In that case, the Ph<0> output signal would bealmost identical to the ClkREf signal 46. However, the second delaystage 55 may provides a unit of delay, and each delay stage thereaftermay provide the corresponding unit delay. Thus, although Ph<0> signal issubstantially in phase with ClkREF signal 46, each of the other outputsignals Ph<1> . . . . Ph<n> would be delayed versions of the input Refclock 46. For example, the Ph<1> output signal will be a unit-delayedversion of ClkREF 46 as compared to the zero delay in Ph<0> output.Similarly, the Ph<n−1> output signal will be an n−1 times (unit) delayedversion of the ClkREF signal 46, and so on. Alternatively, each delaystage 54-57 may provide a unit delay. In that case, the Ph<0> signal isa unit-delayed version of Ref clock 46, the Ph<1> signal is the Refclock 46 delayed by two unit delays, and so on. It is observed here thata dummy delay stage 58 may be included in the VCDL design to matchoutput loading (i.e., loading at the output of the VCDL unit 32) so asto obtain uniform unit delays at each delay stage 54-57 in the VCDL unit32. In the absence of such dummy delay stage 58, the “unit delay” of thefirst delay stage 54 (or the next delay stage 55) may be more than the“unit delay” of the last delay stage 57, which may not be desirable. Itis noted that, in one embodiment, the dummy delay stage 58 may remain“applied” to the input reference clock 46 irrespective of the frequencyof the Ref clock 46. That is, the dummy stage 58 may not be selectivelyturned ON and OFF as are the other delay stages 54-57 (as discussedlater hereinbelow). In any event, an output signal (similar to Ph<0>,Ph<1>, etc.) may not be obtained from the dummy delay stage 58 asindicated in FIG. 4.

FIG. 5 depicts how respective phase signals are generated and input tothe phase detector 30 in a 2-phase analog DLL according to oneembodiment of the present disclosure. The circuit elements shown in FIG.5 may be part of the analog DLL 48 in FIG. 3 when the DLL 48 isconfigured to be operated as a 2-phase DLL. It is noted that only aportion of the analog DLL is shown in FIG. 5, with additional circuitelements (e.g., the bias generator 40 and the VCDL unit 50) omitted forthe sake of clarity. In FIG. 5, the RST signal (at line 72), whenenabled, resets the analog DLL. On the other hand, when the RST signalis disabled, the Init signal (initialization signal) may be enabled toinitialize the Vctrl signal 38 (through a Vctrl Init unit 70) forseveral clock cycles. The Vctrl initializing circuit (e.g., the VctrlInit unit 70) may be desirable to obtain better locking speed. With theinitialized Vctrl 38 (and with reference clock 46 being applied to theVCDL unit 50), the output phase signals, Ph<0:n>, may be obtained at theoutput 52 (FIG. 3) of VCDL unit 50. A portion of these initial outputsignals may be fed to a frequency-to-delay detector (FDD) unit 68. Inthe embodiment of FIG. 5, during initialization, the FDD unit 68(discussed in greater detail in conjunction with FIG. 10) is checkingthe relationship between the frequency of the reference clock 46(represented by the Ph<0> output signal) and the delayed versions Ph<k>of the reference clock 46 (where k=2,4,6, . . . ) to determine theoptimized VCDL stages for the given reference clock frequency. In theembodiment of FIG. 5, n=6. Therefore, the FDD unit 68 receives Ph<0>,Ph<2>, Ph<4>, and Ph<6> signals for detection. With the optimized VCDL,the locking time may be fast with a wide range of input (reference)clock frequencies.

It is observed here that, at the time of design of the analog DLL 48,the number (“n”) of “active” VCDL delay stages (i.e., the dummy delaystage 58 excluded) may be predetermined depending on various designconsiderations including, for example, the range of frequencies that maybe encountered by the DLL 48 for the input reference clock (the desiredlocking range), the unit delay that may be obtained at each delay stage(so as to determine the total delay that may be needed to accommodatethe range of input clock frequencies), etc. Thus, for example, in theembodiment of FIG. 5, n=6, which means that there are seven (7) “active”delay stages and one dummy delay stage in the VCDL unit of the DLL 48.Although the total number of delay stages are fixed and predetermined atthe time of design of the DLL 48, the delay stages in the DLL 48 may beselectively activated (i.e., delay stages that are not required to beapplied to the reference clock may be turned OFF or deactivated, forexample, to conserve power).

The FDD unit 68 operates on the input phase signals Ph<0>, Ph<2>, Ph<4>,and Ph<6> to generate a set of detection signals FDD<0:2> at its output66. Again, a single reference numeral “66” is used herein to refer to agroup of three separate detection signals for ease of reference only. Inpractice, in the embodiment of FIG. 5, the output of FDD unit 68 mayconstitute three separate output lines (not shown in FIG. 5) to carryeach corresponding detection signal. The detection signals FDD<0:2> maybe applied to a group of multiplexers 62, 64 as control inputs to selectan appropriate one of the phase signals input to the multiplexers 62,64. Thus, in case of the multiplexer 62, one of the phase signals Ph<2>,Ph<4> and Ph<6> may be selected to be applied to the phase detector 30as the Ph360 input. Whereas, in case of the multiplexer 64, one of thephase signals Ph<1>, Ph<2> and Ph<3> may be selected as the Ph180 outputof the DLL 48. Thus, the Ph<0>, Ph<180>, and Ph<360> outputs may beobtained from the DLL 48, more specifically, at output 52 (FIG. 3) ofthe VCDL unit 50. The Ph0 input of the phase detector 30 is obtainedthrough a multiplexer 60 having a separate Enable (En) and Disable (Dis)signals at its control input 65.

From the above discussion, it is seen that, in the embodiment of FIG. 5,the generation of the phase signals Ph360 and Ph180 depends on aselection of appropriate one of the VCDL output phase signals Ph<1>through Ph<6> for each of the phase signals Ph360 and Ph180. Further,the selection process is controlled by the outputs from the FDD unit 68,which outputs, in turn, are generated based on the detection of phaserelation between two or more phase signals output from the VCDL unit 50as can be seen from the block diagram in FIG. 5. As discussed withreference to FIG. 9, unneeded or unused VCDL stages may be disabled withoutputs from the FDD unit (e.g., FDD<0:2> in FIG. 5) to save current. Anexemplary circuit layout for an FDD unit to generate various FDD outputsis illustrated in FIG. 10.

FIG. 6 is an exemplary timing diagram illustrating a timing relationshipamong a set of low frequency output signals and the Vctrl signal 38 inthe analog DLL 48 of FIG. 3. In FIG. 6, the input reference clock is alow frequency signal, resulting in low frequency phase output signalsfrom the VCDL unit 50. An even numbered phase output signals (Ph<k>,k=0,2,4,6) are illustrated in FIG. 6. As discussed hereinbefore withreference to FIG. 5, these output signals are also fed into the FDD unit68 to detect the phase relationship between the reference clock (thePh<0> signal) and all other delayed versions thereof (i.e., Ph<2>, Ph<4>and Ph<6> signals in FIGS. 5 and 6). The “high” (logic “1”) and “low”(logic “0”) values for different signals are also indicated withreference to specific instants in time. The waveform for the Vctrlsignal 38 over time is also depicted for the reference. Based on thetiming relationship depicted in FIG. 6 among the phase signals Ph<0>,Ph<2>, Ph<4>, and Ph<6>, it is seen that the FDD unit 68 may beconfigured to select (through appropriate logic value on each of the FDDoutput FDD<0:2> controlling the multiplexers 62 and 64 shown in FIG. 5)either of the Ph<4> and Ph<6> signals to function as the Ph360 input tothe phase detector 30 (FIG. 5), whereas either of the Ph<2> and Ph<3>(not shown in FIG. 6) signals may be selected to function as the Ph180signal (FIG. 5).

FIG. 7 is an exemplary timing diagram depicting a timing relationshipamong a set of high frequency output signals and the Vctrl signal 38 inthe analog DLL 48 of FIG. 3. In contrast to the waveforms in FIG. 6, theinput reference clock in the embodiment of FIG. 7 is a higher frequencysignal, resulting in high frequency phase output signals from the VCDLunit 50 (as can be seen from a simple visual comparison of the period ofphase signals in FIGS. 6 and 7). As in FIG. 6, even numbered phaseoutput signals (Ph<k>, k=0,2,4,6) are illustrated in FIG. 7. The “high”(logic “1”) and “low” (logic “0”) values for different signals are alsoindicated with reference to specific instants in time. The waveform forthe Vctrl signal 38 over time is also depicted for the reference. Basedon the timing relationship depicted in FIG. 7 among the phase signalsPh<0>, Ph<2>, Ph<4>, and Ph<6>, it is seen that the FDD unit 68 may beconfigured to select either of the Ph<2> and Ph<4> signals to functionas the Ph360 input to the phase detector 30 (FIG. 5), whereas either ofthe Ph<2> and Ph<1> (not shown in FIG. 7) signals may be selected tofunction as the Ph180 signal (FIG. 5).

FIG. 8 depicts how respective phase signals are generated and input tothe phase detector 30 in a 4-phase analog DLL according to oneembodiment of the present disclosure. The circuit elements shown in FIG.8 may be part of the analog DLL 48 in FIG. 3 when the DLL 48 isconfigured to be operated as a 4-phase DLL (i.e., having Ph0, Ph90,Ph180, and Ph270 outputs, in addition to the Ph360 output). A comparisonof the embodiments in FIGS. 5 and 8 shows the substantial similaritiesbetween the two block diagrams. However, there are several noticeabledifferences between FIGS. 5 and 8: (1) In FIG. 8, two additionalmultiplexers 74 and 76 are provided to generate outputs Ph180 andPh270), (2) the phase signals (output from the delay stages in the VCDLunit 50) selected as inputs to the FDD unit 68 in FIG. 8 are differentfrom those in FIG. 5, (3) the phase signals input to multiplexers 62, 64in FIG. 8 are different from those in FIG. 5, (4) the multiplexer 64 inFIG. 8 generates the Ph90 output as opposed to the Ph180 output in theembodiment of FIG. 5, and (5) in the embodiment of FIG. 8, n=12 (i.e.,the VCDL unit 50 in the analog DLL 48 includes 13 delay stagesgenerating output signals Ph<0> through Ph<12> and a dummy delay stage).Despite these differences, the construction and operation of theexemplary circuit configuration in FIG. 8 is substantially similar tothe exemplary configuration in FIG. 5 and, hence, no additionaldiscussion of the diagram in FIG. 8 is provided herein.

FIG. 9 is an exemplary circuit layout illustrating how eight enablesignals VEn<0:7> may be generated using outputs FDD <0:3> from an FDDunit (similar to the FDD unit 68 in FIG. 5 or 8) to enable/disable eachdelay stage in a VCDL according to one embodiment having eight delaystages (e.g., the VCDL 50 in FIG. 4 with n=7). In the circuitconfiguration of FIG. 9, the outputs of the FDD unit are shownindividually as inputs FDD<0>, FDD<1>, FDD<2> and FDD<3>, which generateeight enable signals collectively designated in FIG. 9 as signals VEn<0:7> (although each VEn signal may be output over a separate outputline as is known in the art). Each enable signal may be applied to acorresponding VCDL delay stage. For example, the VEn<0> signal may beapplied to the first delay stage (e.g., the delay stage 54 in FIG. 4),the VEn<1> signal may be applied to the second delay stage (e.g., thedelay stage 55 in FIG. 4), and so on. Thus, it is seen from the circuitconfiguration in FIG. 9, that the enable signals VEn may be used toactivate/deactivate individual delay stages in the VCDL unit 50. Forexample, assuming a range of 1 ns to 4 ns clock periods for the inputreference clock 46 (i.e., t_(CK)=clock period of the reference clock=1ns-4 ns) and further assuming that the analog DLL 48 incorporating theVCDL unit 50 receiving such a range of clock frequencies is designed tohave eight (8) VCDL stages (n=7) to provide a delay equal to the maximumclock period of 4 ns, then in the event that the input reference clockperiod is 2 ns (t_(CK)=2 ns, i.e., a higher clock frequency), theVEn<4:7> outputs may be used to disable those four delay stages in theVCDL unit 50 that generate the output signals Ph<4:7> whereas VEn<0:3>may be used to enable or activate delay stages corresponding to outputsignals Ph<0:3> (because n=8). Similarly, if the input clock has a stillhigher frequency (e.g., t_(CK)=1 ns), then VEn<2:7> signals may be usedto disable those six (6) delay stages in the VCDL unit 50 that generatethe output signals Ph<2:7> whereas VEn<0:1> may be used to enablerespective outputs Ph<0:1>, and so on. Thus, selective activation anddeactivation of VCDL delay stages (to accommodate higher and lowerfrequency input reference clocks) may be accomplished with VEn signals,which may be generated using the outputs from an FDD unit (e.g., the FDDunit 68 in FIG. 5 or 8) as depicted in the exemplary circuit of FIG. 9.

As another example of utility of selective activation/deactivation ofdelay stages, it is assumed, as before in the discussion with referenceto FIG. 2, that each delay stage 54-57 in the VCDL unit 50 provides adelay in the range of 300 ps-700 ps. Thus, if the analog DLL 48 (FIG. 3)is designed with four such delay stages in its VCDL unit 50, then allfour stages, when active, may provide a delay in the range of 1.2 ns-2.8ns. However, this range of delay may not accommodate input referenceclock frequency of 1 ns when all four of the delay stages are turned ON(as in case of the prior art DLL 28 in FIG. 2). Further, once the analogDLL 48 is designed, it may not be possible to add or remove delay stagesfrom its VCDL unit 50 during run time. In that event, appropriate VEnsignals according to the teachings of the present disclosure may be usedto deactivate one of the four delay stages while keeping the first threedelay stages active or turned ON so as to achieve a delay range of 0.9ns-2.1 ns, which would not only accommodate the higher input clockfrequency (t_(CK)=1 ns) but would also lower current consumption athigher frequencies (because of deactivation of one or more delaystages). Thus, VEn signals may be used in this manner to control thenumber of delay stages that may be active in the VCDL unit 50 at anygiven time, thereby allowing the DLL 48 to operate with a broader inputreference clock frequency range.

FIG. 10 shows an exemplary circuit layout of an FDD 80 according to oneembodiment of the present disclosure. In the embodiment of FIG. 10, theFDD 80 receives four phase signals Ph<0:3> (outputs of the correspondingVCDL unit, e.g., the VCDL unit 50 with n=3) and also generates fouroutput signals FDD<0:3>. The RST and Initf signals are similar to theRST and Init signals, respectively, shown in FIGS. 5 and 8. The FDD 80in FIG. 10 is also shown to receive the reference clock as the Refinput. Other signals shown in FIG. 10 are generated internal to the FDD80 and are self-explanatory in view of the circuit configuration in FIG.10.

FIG. 11 depicts an exemplary circuit layout according to one embodimentof present disclosure showing how a Ph180 phase signal may be generatedusing outputs of an FDD (e.g., the outputs FDD<0:3> of the FDD 80 inFIG. 10). A circuit configuration similar to the one shown in FIG. 11may be used to generate the Ph180 signal in the configurations of FIGS.5 and 8. Because of the self-explanatory nature of the circuit layout inFIG. 11, no further discussion of FIG. 11 is provided herein.

FIG. 12 shows a table 90 listing a set of clock timing ratios andcorresponding values of FDD outputs (FDD<0:3>) according to oneembodiment of the present disclosure. In the table 90, the terms “t_(D)”and “t_(CK)” have the meanings attributed to them hereinbefore. Thus,the term “t_(D)” denotes a unit delay of a single delay stage in a VCDLunit (e.g., the VCDL unit 50 in FIG. 4) whereas the term “t_(CK)”denotes the clock period of the input reference clock 46. Referring nowto FIG. 10, it is seen that there are four flip-flops receiving thePh<0:3> input lines. These flip-flops are referred to by the term“Latch” in the second column from right in table 90. Thus, “Latch 2”refers to that flip-flop in FIG. 10 which receives Ph<2> signal as inputand generates the e<0> output signal related to the 2 t_(D)/t_(CK)timing ratio (represented by the Ph<2> signal). Similarly, “Latch 4”refers to that flip-flop in FIG. 10 which receives Ph<4> signal as inputand generates the e<1> output signal related to the 4 t_(D)/t_(CK)timing ratio (represented by the Ph<4> signal). And, the “Latch 6” entryunder the “Latch” column in table 90 refers to that flip-flop in FIG. 10which receives Ph<6> signal as input and generates the e<2> outputsignal related to the 6 t_(D)/t_(CK) timing ratio (represented by thePh<6> signal). The fourth flip-flop in FIG. 10 (which generates the e<3>output signal) may receive the Ph<0> signal as input. It is noted herethat the actual signal inputs Ph<2>, Ph<4>, Ph<6> and Ph<0> in FIG. 10are designated as Ph<0:3> input lines, respectively, at the topleft-hand corner in FIG. 10.

FIG. 13 is a block diagram depicting a system 100 in which the analogDLL 48 of FIG. 3 may be used. The system 100 may include a dataprocessing unit or computing unit 102 that includes a processor 104 forperforming various computing functions, such as executing specificsoftware to perform specific calculations or data processing tasks. Thecomputing unit 102 may also include memory devices 106 that are incommunication with the processor 104 through a bus 108. The bus 108 mayinclude an address bus (not shown), a data bus (not shown), and acontrol bus (not shown). Each of the memory device 106 can be a dynamicrandom access memory (DRAM) chip or another type of memory circuits suchas SRAM (Static Random Access Memory) chip or Flash memory. Furthermore,the DRAM could be a synchronous DRAM commonly referred to as SGRAM(Synchronous Graphics Random Access Memory), SDRAM (Synchronous DynamicRandom Access Memory), SDRAM II, or DDR SDRAM (Double Data Rate SDRAM),as well as Synchlink or Rambus DRAMs. Those of ordinary skill in the artwill readily recognize that a memory device 106 of FIG. 13 is simplifiedto illustrate one embodiment of a memory device and is not intended tobe a detailed illustration of all of the features of a typical memorychip. The processor 104 can perform a plurality of functions based oninformation and data stored in the memory devices 106. The processor 104can be a microprocessor, digital signal processor, embedded processor,micro-controller, dedicated memory test chip, or the like.

Each of the memory devices 106 may have construction similar to thatshown in FIG. 1, with the exception that the DLL unit 28 of the priorart is replaced by the analog DLL 48 (FIG. 3) according to oneembodiment of the present disclosure. A memory controller 110 controlsdata communication to and from the memory devices 106 in response tocontrol signals (not shown) received from the processor 104 over the bus112. The memory controller 110 may include a command decode circuit (notshown). The command decode circuit may receive the input control signals(on the bus 112) (not shown) to determine the modes of operation of oneor more of the memory devices 106. Some examples of the input signals orcontrol signals (not shown in FIG. 13) on the bus 112 (and also on thebus 108) include an External Clock signal, a Chip Select signal, a RowAccess Strobe signal, a Column Access Strobe signal, a Write Enablesignal, etc.

The system 100 may include one or more input devices 114 (e.g., akeyboard, a mouse, etc.) connected to the computing unit 102 to allow auser to manually input data, instructions, etc., to operate thecomputing unit 102. One or more output devices 116 connected to thecomputing unit 102 may also be provided as part of the system 100 todisplay or otherwise output data generated by the processor 104.Examples of output devices 116 include printers, video terminals orvideo display units (VDUs). In one embodiment, the system 100 alsoincludes one or more data storage devices 118 connected to the dataprocessing unit 102 to allow the processor 104 to store data in orretrieve data from internal or external storage media (not shown).Examples of typical data storage devices 118 include drives that accepthard and floppy disks, CD-ROMs (compact disk read-only memories), andtape cassettes.

It is noted that the analog DLL 48 according to one embodiment of thepresent disclosure may receive reference clock frequencies in the rangeof 800 MHz-1 GHz. In that case, a small clock jitter may distort theduty cycle. Therefore, it may be preferable to utilize a reference clockfrequency with 50% duty cycle. Furthermore, it is observed that the DLL48 may be designed to be free from stability considerations, unless thefeedback delay is extremely large.

The foregoing describes a methodology to devise an analog delay lockedloop (DLL) or phase locked loop (PLL) wherein the delay stages may beprogrammed according to the operating condition, which may depend on thefrequency of the input reference clock. The resulting optimized delaystages allow for broad frequency range of operation, fast locking timeover a wide range of input clock frequencies, and a lower currentconsumption at high clock frequencies. Better performance is achieved byallowing the number of analog delay stages active during a givenoperation to be flexibly set. The deactivation or turning off of unuseddelay stages conserves power at higher frequencies. The high frequencyrange of operation is increased because of the removal of the prior artrestriction of using a fixed number of delay stages for all input clockfrequencies.

While the disclosure has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the embodiments. Thus, it isintended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

1. A method of operating a synchronous circuit, comprising: applying areference clock as an input to a voltage controlled frequency monitoringunit in said synchronous circuit, wherein said frequency monitoring unitincludes a plurality of operating stages; obtaining a plurality ofoutput signals at an output of said frequency monitoring unit; and usinga first subset of said plurality of output signals to determine whichone or more of said plurality of operating stages in said synchronouscircuit are to be turned on to impart a delay to said reference clock.2. The method of claim 1, further comprising turning off one or more ofsaid plurality of operating stages using said first subset of saidplurality of output signals.
 3. The method of claim 2, wherein saidturning off comprises: generating a set of detection signals based on aphase relation between one or more pairs of output signals in said firstsubset of output signals; and deactivating said one or more of saidplurality of delay stages using said set of detection signals.
 4. Themethod of claim 2, further comprising: generating a first phase signalthat is substantially in phase with said reference clock and a secondphase signal that is substantially 360° out of phase with said referenceclock using a second subset of said plurality of output signals;detecting a phase difference between said first phase signal and saidsecond phase signal; generating a control voltage based on said phasedifference; generating a bias signal based on said control voltage;applying said reference clock and said bias signal to each of saidplurality of delay stages; and obtaining a different one of saidplurality of output signals at a corresponding output of eachnon-disabled delay stage in said plurality of delay stages.
 5. Themethod of claim 1, wherein obtaining said plurality of output signalscomprises: detecting a phase difference between a first phase signalthat is substantially in phase with said reference clock and a secondphase signal that is substantially 360° out of phase with said referenceclock; generating a control voltage based on said phase difference;generating a pair of bias voltages based on said control voltage;applying said reference clock and said pair of bias voltages to each ofsaid plurality of delay stages; activating each of said plurality ofdelay stages; and obtaining a different one of said plurality of outputsignals at a corresponding output of each of said plurality of delaystages.
 6. The method of claim 1, further comprising: generating a firstphase signal that is substantially in phase with said reference clockusing a second subset of said plurality of output signals; andgenerating a second phase signal that is substantially 360° out of phasewith said reference clock using a third subset of said plurality ofoutput signals.
 7. The method of claim 6, further comprising at leastone of the following: generating a third phase signal that issubstantially 90° out of phase with said reference clock using a fourthsubset of said plurality of output signals; generating a fourth phasesignal that is substantially 180° out of phase with said reference clockusing a fifth subset of said plurality of output signals; and generatinga fifth phase signal that is substantially 270° out of phase with saidreference clock using a sixth subset of said plurality of outputsignals.
 8. The method of claim 6, wherein generating said plurality ofoutput signals comprises: generating said plurality of output signalsusing said reference clock, said first phase signal, and said secondphase signal.
 9. A method of operating a synchronous circuit having avoltage controlled frequency monitoring unit with a plurality ofoperating stages, said method comprising: applying a reference clock asan input to said voltage controlled frequency monitoring unit; obtaininga plurality of output signals at an output of said voltage controlledfrequency monitoring unit; checking a relationship between the frequencyof said reference clock and a portion of said plurality of outputsignals to determine a number of operating stages that is optimized forsaid reference clock; and enabling the operating stages that areoptimized for said reference clock.
 10. The method of claim 9, whereinsaid enabling the operating stages that are optimized for said referenceclock includes generating an enable signal corresponding to eachoperating stage to be enabled.
 11. The method of claim 9, furthercomprising: disabling the operating stages that are not optimized forsaid reference clock.
 12. The method of claim 11, wherein said disablingthe operating stages that are not optimized for said reference clockincludes generating a disable signal corresponding to each operatingstage to be disabled.
 13. The method of claim 9, wherein the number ofoperating stages enabled to accommodate said reference clock is lessthan a number of operating stages enabled to accommodate a secondreference clock, where said first reference clock has a higher frequencythan said second reference clock.
 14. The method of claim 9, whereinapplying said reference clock includes applying said reference clock toeach operating stage in said plurality of operating stages.
 15. Themethod of claim 9, wherein said obtaining said plurality of outputsignals includes obtaining a different one of said plurality of outputsignals at a corresponding output of a different one of said pluralityof delay stages.
 16. A method of operating a synchronous circuit havinga voltage controlled frequency monitoring unit with a plurality ofoperating stages, the improvement comprising: turning off one or more ofsaid plurality of operating stages based on an operating condition ofsaid voltage controlled frequency monitoring unit.
 17. The method ofclaim 16, wherein said operating condition of said voltage controlledfrequency monitoring unit includes the frequency of a reference clockapplied to said voltage controlled frequency monitoring unit.
 18. Themethod of claim 16, wherein said operating condition of said voltagecontrolled frequency monitoring unit includes the phase differencebetween a reference clock applied to said voltage controlled frequencymonitoring unit and a plurality of phase signals produced by saidvoltage controlled frequency monitoring unit.